Note that the configure program really does a bad job on C++ code,
because the function exp2() exists but Configure thinks that it does not
exist. This causes paricom.h to declare exp2() in a different way from
<math.h> which then gives an error:

In file included from ../src/headers/pari.h:50:0,
from ../src/gp/gp.c:21:
../src/headers/paricom.h: In function 'double exp2(double)':
../src/headers/paricom.h:94:33: error: new declaration 'double exp2(double)'